Endless belt type continuously variable transmissions (CVT) are used in many types of vehicles such as snowmobiles, all terrain vehicles (ATVs), scooters and the like to obtain an infinite number of gear ratios between the engine and the vehicle's wheels. Typically, the CVTs are mechanically controlled by means of centrifugal weights (usually on the driving side) acting against the force of a spring to provide the desired gear ratios. CVT characteristics, including variation of the gear ratio as a function of engine speed, vehicle speed, torque and the like, can be tailored by appropriate choice of flyweights and spring on the driving side, and the cams on the driven side.
Mechanically controlled CVTs have limited control options as the configuration of the different components cannot be changed on-the-fly based on operating conditions. Mechanically controlled CVTs can have poor power efficiency due to frictional losses arising from the belted construction. Furthermore, when driving at constant high speeds, the engine, and other related elements inside the engine, operating at high rotational speeds (RPM) cause undesirable noise and vibrations, as well as high fuel consumption.
In some CVTs, especially in stationary applications, the mechanical controls have been replaced by a pneumatic or hydraulic system for changing the CVT gear ratio. Schrodt et al., in U.S. Pat. No. 3,200,666, issued Aug. 17, 1965 describes a CVT in which the driving pulley is acted upon by a piston to change the CVT gear ratio. U.S. Pat. No. 6,811,504, issued Nov. 2, 2004 to Korenjak et al., describes a CVT provided with a pneumatic system on the driven side to facilitate braking.
Such fully controlled CVTs, where one or both of the pulleys is controlled, whether hydraulically, pneumatically or electrically, require the attachment of additional components to the CVT, such as pumps and reservoirs, adding to its weight and cost. Furthermore, in such systems any loss of CVT control could result in the vehicle becoming inoperable.
There is thus a need for a CVT that is robust, reliable, efficient and versatile, and which allows for adjustments of its configuration based on vehicle operating conditions.